Field of the Invention
The present invention concerns a method to optimize a magnetic resonance pulse sequence that includes a slice-selection gradient pulse and a gradient spoiler pulse, for operating a magnetic resonance system. Moreover, the invention concerns a method to operate a magnetic resonance system using such an optimized pulse sequence, and a pulse sequence optimization device for a magnetic resonance system in order to implement this method.
Description of the Prior Art
In a magnetic resonance system (also called a magnetic resonance tomography system), the body to be examined is typically exposed to a relatively high basic magnetic field (for example of 1, 5, 3 or 7 Tesla) with the use of a basic field magnet system. A magnetic field gradient is additionally applied by a gradient system. Radio-frequency excitation signals (RF signals) are then emitted by a radio-frequency transmission system by means of suitable antennas, which causes nuclear spins of specific atoms, excited to resonance by this radio-frequency field, to be flipped by a defined flip angle relative to the magnetic field lines of the basic magnetic field. Upon relaxation of the nuclear spins, radio-frequency signals (known as magnetic resonance signals) are radiated that are received by suitable reception antennas, and are then processed further. Finally, the desired image data can be reconstructed from the raw data acquired in such a manner.
For a defined measurement, a defined pulse sequence must be emitted, which includes a series of radio-frequency pulses (in particular excitation pulses and refocusing pulses) and gradient pulses that are emitted in coordination in different spatial directions, as well as suitably placed readout windows during which the induced magnetic resonance signals are detected. The timing within the sequence—i.e. the time intervals at which pulses follow one another—is particularly significant to the imaging. A number of control parameters are normally defined in a measurement protocol, which is created in advance and, for a defined measurement, can be retrieved (from a memory, for example) and possibly modified on site by the operator, who can predetermine additional control parameters (for example a specific slice interval of a stack of slices to be measured, a slice thickness etc.). A pulse sequence (which is also designated as a measurement sequence) is then calculated on the basis of all of these control parameters.
The gradient pulses are defined by their gradient amplitudes, the gradient pulse duration and their edge steepness dG/dt (typically also designated as a “slew rate”). An additional important gradient pulse variable is the gradient pulse moment that is defined by the integral of the amplitude over time.
During a pulse sequence, the magnetic gradient coils with which the gradient pulses are emitted are switched frequently. Eddy currents that are then produced in other components of the magnetic resonance tomography (in particular the radio-frequency shield) and interact with the magnetic fields to produce Lorentz forces, which are one reason for the well-known noise generation during the switching of the gradients. In particular, a high edge steepness contributes to the noise exposure. In addition, steep edges lead to a higher power consumption and additionally place higher demands on the hardware. The rapidly changing gradient fields lead to distortions and oscillations in the gradient coils and to the transmission of these energies to the apparatus housing.
In order to reduce the noise exposure, different solutions have been proposed in the design of the hardware, for example potting or vacuum-sealing the gradient coils. Another possibility is to pay particular attention to the gradient curve already in the calculation of the pulse sequences. In practice, there are therefore apparatuses that offer what are known as different “gradient modes”. The operator can choose between a normal mode and a particularly quiet gradient mode, for example. In the quiet gradient mode, a maximum allowable edge steepness for the gradient pulses is set to a lower value, which leads to the situation that the measurement is quieter than in a normal mode. A disadvantage in the selection of such a quiet gradient mode is that not only is the measurement time made longer overall, but also the image quality (for example the contrast and/or the resolution) is reduced. Given such a limitation of the overall maximum slew rate, a compromise must always be found between the reduction of the noise volume, the measurement time and the image quality. For example, a longer echo spacing (thus a longer interval between the echoes) thus has a negative effect on the contrast and the image sharpness.
In a number of pulse sequences that are used in clinical magnetic resonance tomography (MRT)—for example in spin echo (SE) sequences or turbo spin echo (TSE) sequences—pulses known as gradient spoiler pulses (shortened to spoilers) are activated in addition to the gradient pulses necessary for spatial coding. Gradient spoiler pulses (which are also called gradient crusher pulses—shortened to crushers—in some cases, particularly if they occur in pairs) are emitted by the same gradient coils immediately before and/or after the actual gradient pulses, and ensure that (for example) unwanted free induction decay (FID) signals are suppressed. To ensure suppression of the FID signals with certainty, the spoilers or crushers must have a defined spoiler or crusher moment. In the present application, spoilers or spoiler pulses are normally used, and this designation always includes crushers as well. For differentiation, as used herein spoiler pulses that are emitted before or after slice-selection gradients are designated as gradient spoilers, while spoiler pulses that are executed before or after readout gradients are designated as readout spoilers.
As noted above, the time requirements within a pulse sequence are very strict. In addition, the total duration of a pulse sequence (which determines the total duration of an MRT examination) should be kept as short as possible. Therefore, not much time is provided for the execution of the spoilers. This means that the spoiler amplitudes must necessarily be high to achieve a defined spoiler moment, which leads to high edge steepness of the spoiler pulses directly.
A large part of the noise in MRT examinations, particularly given the use of SE or TSE sequences, therefore results from the spoiler pulses.